Cell for determining the conductivity of liquids entrained in porous media



A. PERANIO April 2, 1968 CELL FOR DETERMINING THE CONDUCTIVITY OF LIQENTRAINED IN POROUS MEDIA Filed Dec.

1 N VENTOR. AZs/r/w/V/ 56 4400 fl'ffole/x ffis United States PatentOfiice 3,3 76 ,5 ill Patented Apr. 2, 1968 3,376,501 CELL FORDETERMINING THE CONDUCTIVITY OF LIQUIDS ENTRAINED 1N POROUS MEDIAAnthony Peranio, Haifa, Israel, assignor to Vanian Associates, PaloAlto, Calif., a corporation of California Continuation-impart ofapplication Ser. No. 556,593, June 10, 1966. This application Dec. 9,1966, Ser. No. 600,581

4 Claims. (Cl. 32430) ABSTRACT OF THE DISCLOSURE A cell for determiningconductivity of liquids comprising a casing having a block of resinousmaterial therein, a cable extending into the casing and block, at leastone pair of electrodes embedded in the block having its surface exposedat a face of the block, wires connecting with the electrodes in theblock, hollow space between the face and the opposite wall of thecasing, a plurality of openings in the casing to allow liquid to enterthe hollow space. The electrodes are spaced from the surroundingmaterials a distance about equal to or greater than the distance betweenthe electrodes themselves.

The present application is a continuation-in-part of application SerialNo. 556,593, filed June 10, 1966, and entitled Conductivity Cell, whichin turn is a continuationin-part of application Serial No. 188,786,filed Apr. 10, 1962, and entitled Cell for Measuring ElectricalConductivities of Liquids, both now abandoned.

This invention relates to a device for determining the electricalconductivity of liquids entrained in soils. It has particularapplication for measuring electrical conductivity of liquids insaturated porous media; for example, determining the salinity ofunderground water in sand.

A technique for measuring the salinity or electrical conductivity ofunderground water necessitates the boring of an observation well to thelevel at which water salinity is to be determined. A standardconductivity cell is lowered into this well to the proper level, withelectrical connections to a bridge circuit (which is the usual case) andfrom the earths surface a determination of water salinity is made. Inanother technique, an evacuated ampule is lowered to the desired depthand from up above it is broken open. Water rushes in and fills theampule. The ampule is then raised to the surface and a salinitydetermination is made, using conventional instrumentation.

Both these techniques are based upon the construction of a suitableobservation well. In addition, an uncertainty exists as to whether ornot the water existing in the observation well pipe is reallyrepresentative of the water outside the pipe at that same depth orlevel. Finally, both techniques are time consuming since they requirelowering and raising either a conductivity cell or an ampule to and froma considerable depth at each observation well.

In the present invention, a specially constructed cell is lowered into adrilled hole mination is to be made. An electrical cable is connected toit from the surface. The hole is then filled in, and just a terminalstrip is left exposed. In this way, no observation well with a specialscreen is necessary. In addition, the cell samples water in the porousmedium at a certain depth. Finally, salinity or conductivity can bechecked quickly using a portable instrument that can be taken into thefield.

In its specific embodiment, the cell is constructed of to the depth atwhich a deterseveral basic elements; a casing made of electricallynonconducting material, a screen inside part of the casing, electrodesand connecting wires, and a cable or cables from outside the casing,passing through it and connected to the electrodes. The casing consistsof a hollow, relatively thin-walled tubular portion provided with disksto close off its end openings. Internally, the casing is divided intotwo semi-cylindrical portions, one of which has a plurality of openingsin its side, and ends to allow free access of water or other fluid. Theother portion holds four electrodes and wire connections therefrom tothe top of the casing where a cable enclosing the wires leads tomeasuring instruments. A solid block of electrically insulating materialfills the second portion embedding the electrodes and the wiring. Onesurface of each of the electrodes is in the face plane of the block andcomes in contact with any fluid flowing into the space of the firstportion. A screen covers the openings in the casing to prevent ingressof solid matter.

The invention is more fully described in connection with theaccompanying drawing, in which like reference characters indicate likeparts and in which:

FIG. 1 is a vertical cross-sectional view of a cell constructed inaccordance with the present invention, some parts being shown inelevation;

F IG. 2 is a top plan view thereof;

FIG. 3 is a vertical cross-sectional view taken at right angles to thatof FIG. 1; and

FIG. 4 is a vertical cross-sectional view of another embodiment of thecell in which some parts are shown in elevation.

Shown in FIG. 1 is a pair of current-carrying electrodes 1 and 2, andpotential measuring electrodes 3 and 4. These electrodes are made of anoble metal, preferably platinum, so that even though they are buriedunderground, or are used in some other corrosive atmosphere, they do notchange significantly in electrical characteristics; especially inresistivity. Cables 5 and 6 are two-conductor cables with electricalscreens or insulation whose outside shield is made of vinyl or otherplastic material of long-term stability suitable for installationunderground. Cable 5 is connected to the current electrodes, and cable 6is connected to the potential electrodes. The entire electrode I theplatinum electrodes. Just the faces of the electrodes are left exposed(the faces showing in FIG. 1). These faces are preferably in alignmentwith one another as shown in FIG. 1. All other parts of the electrodesand all connecting wires are effectively sealed off and insulated by thecast resinblock material.

The electrode assembly of semi-cylindrical shape is glued into arelatively thin-walled tube or pipe 7 of matching radius. Tube 7 isconstructed of material which is substantially electricallynon-conducting, and has sulficient strength to withstand pressureexerted by the soil in which the cell is to be buried. Suitablematerials for this purpose were found to be polyester resin, polyesterresin reinforced with glass fibers, and epoxy resin. A large number ofpossible materials in the class of synthetic resins that polymerize,with or without filler, can be used for this application, and thesematerials are well-known to those versed in the plastics art.

Another manufacturing technique is to cast the electrodes in a singleunit of epoxy resin to the desired shape of both the tubular portion oftube 7 and the block 10. Tube 7 is provided with a series of relativelylarge holes 13 that permit water or other liquid to flow freely into andout of its inner space. Tube 7 is further provided with disk-like endcovers 8 and 9, provided with holes which connect to the tubes innerspace. Since the cell of the invention may be used in relatively fineporous material (such as sand), 21 method has also been provided forpreventing the intrusion of sand or other earth particles into thehollow inside portion of cylindrical casing 14, which casing is made upof tube 7 and end disks 8 and 9. A fine mesh screen 11 is provided allaround the hollow inside of casing 14- except over the face of theelectrodes 1, 2, 3, and 4. The screen acts to prevent solid particlesfrom entering the cell, but permits the free flow of liquid through thecell. The fine mesh screen used in this embodiment of this invention wasmade of polyethylene strands. However, it could be made of any suitableplastic materials. It is important that the screen material bechemically stable, and be a good electrical insulator.

The invention is seen to consist, therefore, of a thinwalled hollowcasing made of electrically non-conducting material having relativelylarge openings that are screened against the intrusion of all butextremely small particles of earth or sand. Inside this casing is placeda set of four chemically stable electrodes fixed into an insulatingblock so that just a single surface of each electrode faces into thehollow enclosure. Passing through the wall of the casing, and connectedto the electrodes, are insulated wires for making connections duringconductivity measurements.

If it is desired to keep track of the salinity of water in an aquifer ata certain location at a depth of one-hundred feet below the surface,this invention may be used as follows: A hole is drilled large enough totake the cell of this invention. The cell is lowered into the hole untilit reaches the desired level by means of its connecting cables. The holeis filled in. Water at the depth of onehundred feet enters the cell. Atthe surface, conductivity measuring apparatus of the four-electrode typeis connected to the cells cables. Readings are obtained, and these arecompared with the results of previous calibration of the cell in thelaboratory. By comparing these readings, the salinity of the water atthe one-hundred foot depth can be determined. This type of measurementis well-known in the art of conductivity measurements and will not bedescribed here.

The conductivity cell of FIG. 4 is identical in all respects with thatshown in FIGS. 1, 2, and 3, except that two electrodes are used insteadof four. This alternative embodiment has the disadvantage that accuracyof measurement is dependent upon the long term stability of its twoelectrodes. Any changes in their resistivity from the originalcalibration will introduce error into the determination. The firstconductivity cell described is relatively free from this effect; thatis, using four electrodes instead of two. Finally, the conductivity cellaccording to this invention has been designed so as to be virtuallyinsensitive to the effect of any surrounding non-conducting medium.

It is known that the conduction path of an electric current flowing in aconductive liquid extends to great distances from the electrodescarrying the current into and out of the liquid. However, for allpractical purposes, the greater part of the current (say 99.5%) iscarried within a liquid space which is finite around the electrodes ofthe cell. Laboratory tests have shown that the influence of materialsother than the conducting fluid upon current flowing between electrodesplaced in a conducting liquid is virtually nil if these other materialsare kept approximately the distance of the electrode spacing from theelectrodes.

This principle has been used in the design of the cells of thisinvention. Thus in these embodiments of the invention, the filter screenand the outside rigid casing have been made of electricallynon-conducting materials.

Further, because a relatively large distance exists between the surfacesof the electrodes and the screen and casing surfaces, these parts havevery little effect upon the cells behavior. In other words, were theelectrodes and cast resin block to be stripped of the screen and casingand placed in a conducting liquid, far from the walls of any container,the reading of the cell would be less than 0.5% I

different than the reading obtained by the complete cell.

This characteristic is of prime importance when the cell is buried inliquid-containing sand, for instance. Tests showed that the size, shape,packing, or other characteristics of the sand had no influence on thecell reading. Virtually the only influence on the cell reading (allother electrical factors being equal) was the electrical conductivity ofthe liquid entrained in the sand sample. This characteristic is also ofimportance if the cell is to be used in a metallic pipe well'whose wallsare in close proximity to the cell. In this case, also, the effect ofany external material or body will not influence the cell readingsignificantly.

The plurality of holes in the casing permits free flow of water or otherfluid entrained in the sand or earth around the cell to circulate freelyin and out of the cell. The screen prevents particles of sand fromentering the cell and interfering with the electrical characteristics ofthe electrodes. The closest elements to the exposed surfaces of theelectrodes are the screen and the casing.

These have deliberately been placed relatively far away from theelectrode surfaces. Due to this fact the existence of other bodiesoutside the casing such as the metallic wall of a well pipe, closelypacked sand, earth, or other bodies, will not affect the electricalcharacteristics of the cell to any appreciable degree. The solid blockof resin into i which the electrodes and wiring have been cast protectsthem from corrosion, and fixes the electrodes mechanit cally.

underground for long periods of time, and it will be imaffected bychanges in sand packing, earth physical composition, and the presence ofmetallic or other foreign bodies. It will operate underground undersevere environmental conditions and will respond only to the electricalconductivity of the water or other fluid flowing through its openingsand screen.

In the claims the term casing volume is intended to means that volumedefined by the inner walls of the casing.

What is claimed is:

1. A cell for determining conductivity of liquids comprising asubstantially thin-walled casing of non-conductive materials, saidcasing defining a casing volume, a

block of resinous, insulating material in said volume and filling aportion thereof, said block having a face, a cable extending into saidcasing the surface of said electrodes exposed at saidface only, spacebetween said face and an opposite wall of said casing, means forallowing access of liquid into said space, a liquid permeable screeningmeans associated with said casing, the surfaces of said electrodes beingspaced from said casing and said screening means by a distance at leastabout equal to the distance between said electrodes, j

said casing having two ends, said block extending from one of said endsto the other of said ends, said electrodes being in alignment, saidblock occupying at least about.

and said block, at least one 1 pair of spaced electrodes embedded insaid block with i References Cited UNITED STATES PATENTS 6/1931 Behr3243O 6/1937 Maass 32430 Christie 324-3O Bouyoucos 32465 X Smith 324-1(lX Sproule 324-30 5 RUDOLPH v. ROLINEC, Primary Examiner.

C. F. ROBERTS, Assistant Examiner.

UNITED STATES PATENT OFFICE CERTIFICATE OF CORRECTION Patent No.3,376,501 April 2, 1968 Anthony Peranio It is certified that errorappears in the above identified patent and that said Letters Patent arehereby corrected as shown below:

In the heading to the printed specification, lines 4 and 5, for "AnthonyPeranio, Haifa, Israel, assignor to Varian Associates, Pal-0 Alto,Calif., a corporation of California" read Anthony Peranio, 18 RuthStreet,

Haifa, Israel Signed and sealed this 8th day of July 1969.

(SEAL) Edward M. Fletcher, Jr.

Commissioner of Patents Attesting Officer

